1. Field of the Invention
The present invention relates to an ischemia detector of the type wherein repolarization of the heart of a patient is sensed and corresponding repolarization signals are analyzed to detect an ischemic state (ischemic episode).
2. Description of the Prior Art
Ischemia is a condition resulting from insufficient blood flow through the heart muscle. The reason therefor is blocking or passage congestion of coronary blood vessels of the heart. An ischemic heart also loses its ability to adapt the heart blood flow to the demand, e.g. the workload. An ischemic episode is experienced by the patient as a severe chest pain and is one of the most stressing factors known to the organism and the patient is normally forced to sit down or lie down and feels the need for forced breathing, so called hyperventilation.
Compensatory effects will be activated to cope with the ischemic situation. Thus the heart rate will increase due to sympathetic nerve stimulation and increased catecholamines stimulation. In case of a limited or local ischemia this activation can compensate for the reduced capacity of a limited ischemic area of the heart.
In U.S. Pat. No. 5,199,428 a technique is described for detecting an ischemic episode and effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart's oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptable limits to avoid bradyarrhythmias and/or unphysiological AV delays induced by the nerve stimulation. The ischemia detection is based on the occurrence of changes in the ST-segment variation different from predetermined or programmed threshold levels, or on changes in the pH and/or in the dissolved blood oxygen in venous return blood in the coronary sinus region of the patient's heart.
An ischemic state can also be detected by an analysis of recorded IECGs or surface ECGs to determine the heart rate variability. An ischemic state can be detected by a lead bending sensor located at the distal end portion of an implanted heart stimulator lead. Because the heart wall becomes thicker and stiffer as the result of ischemia, the accompanying change in the moving pattern of the heart wall can be detected in this way. Also, sound absorption is effected by changes in the stiffness of the heart tissue and by measuring the absorption of sound waves, generated e.g. at the heart valve closure, as they propagate from the upper portion of the ventricle to the apex region, an ischemic situation can be detected. An ischemic episode deteriorates the efficiency of the pumping of the heart and an ischemic situation therefore can be detected by studying blood pressures and cardiac outputs as well. Thus, by measuring the difference between the systolic and diastolic pressures and comparing this difference obtained from one heartbeat to the difference obtained from the next heartbeat an ischemic state can be detected. With the aid of a flow sensor for measuring cardiac output an ischemic state can be detected as well. An ischemic state also can be detected from the occurrence of the abnormal combination of a low workload and high breathing activity, which is typical of ischemic patients.
The onset of an increased heart rate related to an ischemic situation also can be detected from changes in the repolarization of the heart, such as changes in the QT interval, T-wave amplitude etc.
From U.S. Pat. No. 5,330,511 it is known to study variations in the QT-interval for determining an optimized AV-interval for a predetermined pacing rate for the control of a dual chamber pacemaker. The variation of the QT-interval is then studied as a function of the AV-delay for a fixed pacing rate and an optimum AV-interval is determined that interval which corresponds to the maximum QT. In U.S. Pat. No. 4,228,803 a physiologically adaptive cardiac pacemaker is disclosed having a circuitry for measuring the time interval between a stimulus pulse and the following T-wave. The escape interval of the pacemaker pulse generator is varied in accordance with the detected stimulus-to-T-wave interval so as to vary the pacing rate in accordance with this interval variation. Since this interval corresponds to physiological changes, the pacemaker is adapted to automatically follow the patient's physiological changes.